12V 10Ah SLA Charging problem

hippy

hippy

Technical Support
Staff member
For clarity and comprehension I would have drawn the PSU like this ...

Code: 
CHG + >--------------------.--|>|--.---.---> 12V
CHG - >---.      __        |       |   |
          |  .--|__|--|<|--'       |   |
          |  |   10                |   `---> LOAD +
BAT + <>--|--^-----.----------|>|--'   .---> LOAD -
BAT - <>--|       .|.                  |
          |   47K |_|                __|__
          |        |                |     |
          |        }--->        >---| FET |
          |       .|.               |_____|
          |   10K |_|                  |
          |        |                   |
         -^-      -^-                 -^-
 
lbenson

lbenson

Senior Member
Good point about reading the current from the 12V battery.

I was afraid to test initially with a Pi-ZW but had an extra NEXX wt3020 micro router set up running openWrt. I set up a tiny adjustable switch-mode module ( https://www.ebay.com/itm/DC-6-5-28V-to-DC-5V-MP1584-3A-DC-DC-Mini-Step-down-BUCK-Module-HS/372104494361 ), soldered on a USB breakout board, adjusted to 5V, connected to my LOAD pins, and plugged in the nexx. It came up and I was able to connect to it. The voltage shown on my DVM was solid. The current (from the larger switch-mode module set at 13.4V) never exceeded 100mA. The Pi-ZW will be a bit more.

On my circuit, I need to move the voltage divider to the left of the diode from the battery. I will add a signal to go the the Pi to tell it that the power will be disconnected in 1 minute so that it can shut down properly.

The Pi-ZW is headless. I'll look into disabling the HDMI interface. I have set up the system (motioneye OS) to take snaps when motion is detected. These are sent to my hub. You can view the streaming video either through the hub (with other cameras) or at the IP of the R-Pi-ZW.

I haven't tested the Pi on a battery yet. A nominal 4W 12V LED has been running on the battery for 4 days, with voltage dropping from 12.8V to 12.0. The same LED on a 425AH UPS ran for 2 hours, a Pi-ZW for about 2 and a quarter. I think the 425AH UPS is overrated--the same LED ran for 4+ hours on a nominal 350AH UPS. But obviously, running directly off of a 12V battery is far superior.


The little switch-mode device comes in a variation with fixed 5V output and USB onboard. I have ordered several of those.
 
premelec

premelec

Senior Member
@ Ibenson Thanks for ref to the project - if you add a small PV solar panel charging the battery you can keep things alive most of the year...
 
lbenson

lbenson

Senior Member
hippy said:
... I would have drawn the PSU like this ...
Click to expand...
Far superior layout, hippy. I have moved the voltage divider so that if mains comes back on after an outage, the PICAXE will know to activate the load immediately without waiting for the battery to charge up. The code provides hysteresis so that the battery bounce of removing the load doesn't trigger premature re-activation.

Code: 
CHG + >--------------------o--|>|-----o---o---> 12V
CHG - >---.      __        |          |   |
          |  .--|__|--|<|--'          |   |
          |  |   10                   |   `---> LOAD +
BAT + <>--|--o--------|>|---o---------'   .---> LOAD -
BAT - <>--o                .|.            |
          |            47K |_|          __|__
          |                 |          |     |
          |                 o--->  >---| FET |
          |                .|.         |_____|
          |            10K |_|            |
          |                 |             |
         -^-               -^-           -^-
 
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